This invention relates to cooling agent mixtures for use in low-temperature freezing processes. This invention also relates to low-cooling agent mixtures which do not corrode stainless steel and aluminum.
The freezing of ice cream, the low-temperature freezing of packaged food, the maufacture of packaged fish fillets, which must be exposed quickly to low temperatures, as well as the low-temperature freezing of coffee and tea for subsequent freeze drying, are performed in different types of freezing equipment. Such equipment is based upon the principle of indirect cooling by circulating cooling agents, such as aqueous salt solutions, which in turn are cooled by an evaporating freezing mixture, such as ammonia or carbon fluoride. The cooling agent transmits the coldness to the material to be frozen.
In the case of immersion freezing, the packaged material to be frozen, which is present in cooling molds or metal pans, is conducted through a brine bath with the aid of transport equipment. The contact freezing process is carried out in periodically operated plate-freezing units. Within the plates consisting of aluminum or stainless steel there are ducts in which the brine is circulated. In continuously operated belt-freezing installations, heat is removed from the belt by spraying with brine.
Aqueous salt solutions of calcium chloride are the most frequently utilized as suitable cooling agents for temperatures down to about -45.degree. C. However, such solutions have a corroding effect on aluminum and stainless steel, and are moreover afflicted with the disadvantage that their viscosity increases at lower temperatures. Because of their more favorable behavior with respect to iron and non-ferrous metals, use is also made of ethylene glycol-water mixtures, but their use is limited by an increasing viscosity in higher concentrations.
In view of the foregoing disadvantages, in particular the corrosion problems, it has become necessary to find a cooling agent which does not possess corroding effects and which, with respect to cooling characteristics, would at least be equal to the known solutions which are based on chlorides, and, furthermore, would meet the requirement for an acceptable viscosity, flash point, and odor.